1. Field of the Invention
This invention relates to a thermal head apparatus, and more particularly to a thermal head apparatus which is suitably applied to a comparatively inexpensive thermal printer of a small size.
2. Description of the Related Art
Conventionally, when a thermal head apparatus of the type mentioned is used to print at a high speed, the temperature of the thermal head itself rises gradually due to a heat accumulating action of the thermal head itself, and as printing proceeds, the printing density increases gradually, resulting in defective printing of collapsing printed characters or elongated printed characters. Therefore, a heat accumulation correction circuit is provided for a printing control circuit in order to allow high speed printing. However, not for character printing but for printing of a shading pattern which includes crowded dots, a large scale control circuit is required. Further, in recent years, a printing method which realizes color printing with thermosensible paper has been developed and put into practical use. In order to perform shading printing with gradations by the printing method, finer temperature control of heat generation elements than ever is required, and conventional thermal head printing control methods cannot always satisfy the requirement sufficiently.
As a solution to the problem, a thermal head apparatus has been proposed wherein resistor elements each having a resistance value which varies depending upon the temperature thereof by heat generated by the same are employed as heat generation elements and are controlled by a control circuit which Includes a plurality of comparatively inexpensive general purpose integrated circuits in order to allow comparatively fine printing temperature control. The thermal head apparatus employs a control method wherein, in a process of driving the heat generation elements, whose resistance values vary depending upon the temperatures thereof, with electric currents to generate heat which causes temperature rises of the heat generation elements, the temperatures of the heat generation elements are detected repetitively and, when a predetermined temperature of a heat generation element is detected, the driving of the heat generation elements with electric current is stopped. The thermal head apparatus described above will be described in more detail below with reference to FIGS. 4, 5 and 6.
Referring first to FIG. 4, a thermal head denoted at 50 includes 64 heat generation elements R1 to R64, a heat generation driving integrated circuit 80 and an electric current detecting integrated circuit 58. The heat generation driving integrated circuit 80 includes a shift register circuit 801, a latch circuit 802, an output gate circuit 803, and 64 output transistors Q1 to Q64. Meanwhile, the electric current detecting integrated circuit 58 includes a shift register circuit 181, a latch circuit 182, an output gate circuit 183, and output transistors q1 to q64. All of the heat generation elements R1 to R64 are connected at one ends thereof to a common electrode 52, to which a dc power source voltage VHD for driving the thermal head apparatus is applied. The other ends of the heat generation elements R1 to R64 are connected to the heat generation driving integrated circuit 80 by way of respective electric current detecting resistors r1 to r64. The other ends of the heat generation elements R1 to R64 are connected also to the electric current detecting integrated circuit 58.
As seen from FIG. 5, print input data Din are inputted in the form of a serial signal together with a synchronizing signal D-Clock to the shift register circuit 801 and then transferred at a time to the latch circuit 802 at the timing of a latch signal D-Latch. The output gate circuit 803 turns on the output transistors Q1 to Q64 in response to the print data transferred to the latch circuit 802 and keeps the on-state of the output transistors Q1 to Q64 for a period of time within which a strobe signal D-Strobe exhibits a low (L) level to flow electric currents through the heat generation elements R1 to R64 to generate heat.
In this instance, the electric currents I1 to I64 flowing through the heat generation elements R1 to R64 substantially depend upon the dc power source voltage VHD and the resistance values of the heat generation elements R1 to R64. Further, since the resistance values of the heat generation elements R1 to R64 vary by a great amount depending upon the temperature, also the flowing electric currents vary by a great amount by heat generation upon printing. In other words, the electric currents I1 to I64 and the temperatures of the heat generation elements R1 to R64 have a correlation, and the temperatures of the heat generation elements R1 to R64 can be detected from the values of the electric currents I1 to I64. Further, the electric currents I1 to I64 have a proportional relationship to the voltages appearing across the electric current detecting resistors r1 to r64. Accordingly, the voltages are extracted to the outside in the form of an external serial signal Sout of the thermal head 10 by way of the electric current detecting integrated circuit 58.
A serial input Sin to the electric current detecting integrated circuit 58 includes data of "1" of a high level only at one bit at the top thereof while the other bits of the serial input Sin exhibit a low level. The serial input Sin is inputted to the shift register circuit 181 in response to a clock signal S-Clock. The data "1" of one bit thus inputted is transferred to the latch circuit 182 at the timing of a latch signal S-Latch. The clock signal S-Clock and the latch signal S-Latch have an equal period but the latch signal S-Latch is delayed a little in timing with respect to the clock signal S-Clock. Thus, as the serial input Sin is successively shifted in the shift register circuit 181, the output transistors q1 to q64 are successively turned on in the reverse order, and consequently, the voltages across the electric current detecting resistors r1 to r64 successively pass, from the electric current detecting resistor r1 side toward the electric current detecting resistor r64 side, through the corresponding output transistors q1 to q64 and outputted to the external serial signal Sout.
Signals corresponding to the electric currents I1 to I64 which have a correlation to the temperatures of the heat generation elements R1 to R64 are extracted from the terminal Sout and transferred to a control circuit 42 shown in FIG. 6 which is provided outside the thermal head 50. Referring now to FIG. 6, in the thermal head 50, the signals are successively converted into digital amounts by an analog to digital (A/D) converter 421 and then compared with a temperature set by a setting unit 423 by a comparator 422. When a temperature represented by any of the signals is lower than the set temperature, a signal of a high (H) level is produced by the comparator 422, but when the temperature is equal to or higher than the set temperature, a signal of a low (L) level is produced. The thus produced signal is fed back to the serial input Din of the thermal head 50. The sequence of operations described above is repeated for each one period of the clock signal D-Clock and the clock signal S-Clock for the heat generation driving integrated circuit 80 and the electric current detecting integrated circuit 58, respectively.
Referring also to FIG. 4, the clock signals for the shift register circuit 801 of the heat generation driving integrated circuit 80 and the shift register circuit 181 of the electric current detecting integrated circuit 58 are synchronized with each other, and the output terminals of the output transistors Q1 to Q64 and q1 to q64 of the integrated circuits 80 and 58 connected to the electric current detecting resistors r1 to r64 are connected to each other such that the terminal numbers of them are reverse to each other in order. Consequently, the signal outputted from the terminal Sout of the electric current detecting integrated circuit 58 coincides with the controlled print data in terms of both of the timing and the sequential order.
For each printing cycle, energy for printing is applied by a plurality of times to the heat generation elements R1 to R64, and the temperatures of the heat generation elements R1 to R64 at the instant of each application are detected. Then, subsequent application of the printing energy to any of the heat generation elements R1 to R64 which exhibits a temperature equal to or higher than the set temperature is stopped. In this instance, print data Datain for the first application time in each printing cycle are transferred from the control circuit 42, but at and after the second application time, data of the shift register circuit 801 are cyclically transferred and used. Such switching is performed in response to a selection signal Select. In this instance, the comparator signal from the comparator 422 is inputted to the serial input Din, and the comparator signal exhibits a high level only at portions thereof corresponding to those of the heat generation elements R1 to R64 whose temperatures are lower than the predetermined temperature. The comparator signal and the output of the shift register circuit 801 are logically ANDed by an AND circuit 44, and the shift register circuit 801 exhibits a high level only at stages thereof corresponding to those of the heat generation elements R1 to R64 whose temperatures are lower than the predetermined temperature. Consequently, energy is applied only to those heat generation elements R1 to R64. Reference numeral 46 denotes a switch (SW) for selectively inputting the output of the AND circuit 44 and the print input data Din to the shift register circuit 801.
The conventional thermal head apparatus described above, however, includes a comparatively large number of integrated circuits in the thermal head since it includes a heat generation driving circuit and a temperature detection circuit separately, and requires a high production cost since electric current detecting resistors are required by a number equal to the number of heat generation elements.
Further, where the conventional thermal head apparatus described above is used to print on a medium which has such a three layer structure of color developing layers for three primary colors as seen, for example, in FIG. 7 and wherein the density in color at a portion thereof contacting with a heat generation element increases for each color as the temperature of the heat generation element rises and the printing density varies in order of yellow, magenta and cyan as the temperature rises, when printing is performed for the cyan color developing layer of the lower layer of the medium, as driving of the heat generation element proceeds, the temperature of the heat generation element of the thermal head rises. However, since the temperature at the surface of the heat generation element and the temperature of the cyan layer of the medium exhibits a difference due to a transmission time of heat in heat transfer between them, before the cyan color developing temperature actually rises to an aimed temperature therefor, it is determined in error that the aimed temperature is reached, and consequently, driving of the heat generation element is stopped, resulting in printing in insufficient density.